Blender Git Loki

Cycles: Adding Hilbert Spiral as a tile order for rendering

This patch adds the "Hilbert Spiral", a custom-designed continuous space-filling curve, as a tile order for rendering in Cycles.
It essentially works by dividing the tiles into tile blocks which are processed in a spiral outwards from the center. Inside each
block, the tiles are processed in a regular Hilbert curve pattern. By rotating that pattern according to the spiral direction,
a continuous curve is obtained, which helps with cache coherency and therefore rendering speed.

The curve is a compromise between the faster-rendering Bottom-to-Top etc. orders and the Center order, which is a bit slower,
but starts with the more important areas. The Hilbert Spiral also starts in the center (unless huge tiles are used) and is still
marginally slower than Bottom-to-Top, but noticeably faster than Center.

Reviewers: sergey, #cycles, dingto

Reviewed By: #cycles, dingto

Subscribers: iscream, gregzaal, sergey, mib2berlin

Differential Revision: https://developer.blender.org/D1166